The present invention relates to an air classifier and in particular to apparatus for separating dust particles from larger particulate or granulate materials.
Air classifiers are known comprising a pipe section concentricly arranged about a displacement body thereby defining an annular space. The pipe section is in turn enclosed by a second pipe section which defines with it a second annulus. The second pipe section projects below the lower end of the first pipe section into the upper part of a receiving container, which is provided near its upper end with an inlet for the classifying air. The bulk material which is to be separated is introduced into the top of the first or inner pipe section and falls downwardly through it, until it reaches the area of the surrounding outer or second pipe, where it is contacted by the incoming classifying air. The classifying air deflects the bulk material separating the dust from it and carrying the dust upward through the second or outer annulus to an outlet.
Such air classifiers are used for instance in the processing of plastic materials, in order to rid the granulates fed to the extruder of dust which for example adhere to the granulates by electrostatic charge. Such dust, carried along with the granulates might otherwise foul the extruder or cause difficulties in production such as forming irregularties in extruded foils or ruptures in the threads spun in the extruder.
In such deflection classifiers, the bulk material to be separated is blown pneumaticly into the classifier and flows through the first annular space by gravity. At the end of the annular space the bulk material first enters the deflecting air stream of classifying air. The volume of air in the stream may be adjusted so that the velocity equals approximately the speed of the falling granulates. The larger particles penetrate the classifying air due to their greater kinetic energy and are collected in the lower receiving container. The fine particles or dust are however braked in their fall, deflected upwards and are carried away by the air stream, through the outer annulus. The separated bulk material is removed from the container by way of a cellular wheel sluice or other discharge device. The dust and fine particles are removed from the air stream by a dust separator.
While the advantage of this principle of separation lies in the fact that the tear-off (detaching) forces between the granulate material and the dust lie in the range which is about 20-fold the detaching forces attainable in a gravity classifier and in addition the relatively high velocity of the bulk material leads to shorter dwell periods and smaller structural sizes in the separator portion, the hourly throughput of the bulk material on the other hand is limited to roughly 20 to 25 tons per hour. Such limits cannot be overcome merely by enlarging the dimensions of the deflection separator. Critical to the maximum throughput is the area of the annular gap between the displacement body and the first or inner pipe section. An enlargement of this gap width will cause the stream of granulate flowing out of the lower end of the first or inner pipe section to be expanded by the current of the classifying air flowing in the opposite direction to such an extent that a part of the granulate grain will impact the wall of the outer second pipe section and will be braked as a result, to an extent that it will be carried along by the air current with the dust portion. An enlargement of the diameters of the displacement body, and the first and second pipe sections, while by maintaining for the reasons cited above the maximum possible width of the annular gap between the displacement body and the first pipe section would also result in a larger cross-sectional area of the first annular chamber and is therefore not possible because an even distribution of the bulk material over the entire area of the annular gap can no longer be guaranteed. The uneven dispersion of the bulk material observed in the conveying air is presumably attributed to the configuration of the pneumatic conveyor or system and in particular to the last pipe end by way of which the bulk material is fed from the conveying conduit into the classifier, and which as is known, causes the formation of strands and groups of material due to the deflection of the stream. These strands of insufficiently dispersed bulk material penetrate the air current in the form of locally increased concentrations, without the dust portions being detached.
It is the object of the present invention to provide an deflection classifier of the type described such that along with the maintenance of the high quality of separation, the maximum quantity of bulk material capable of being conveyed per time unit can be substatially increased.
This object as well as others together with the numerous advantages of the present invention will be apparent from the following disclosure of the present invention.